Viral Encephalitis Causes

Herpes simplex (HSV): This type of virus causes cold sores of the mouth and lesions of the genitals. HSV is transmitted directly through human contact. Newborns can also get the virus by passing through an infected birth canal. Once inside the body, the virus travels through nerve fibers and can cause an infection of the brain. The virus may also undergo a period of latency in which it is inactive. At a later time, emotional or physical stress can reactivate the virus to cause an infection of the brain. It causes the most subacute (between acute and chronic) and chronic (lasting
three or more months) encephalitis infections in humans.

Arbovirus: Hosts are animals such as birds, pigs, chipmunks, and squirrels that carry the virus. Mosquitoes (known as vectors or ways of transmitting the virus) feed on these animals and become infected. The virus grows and cycles between the hosts and the vectors. Humans become infected through mosquito bites. Once inside the body, the virus replicates and travels in the bloodstream. If there is a large enough amount of the virus, the brain can become infected. The majority of cases occur between June and September when the mosquitoes are most active. In warmer climates, the disease can occur year-round.

West Nile virus (WNV): This virus was first isolated from an adult woman with a fever in the West Nile District of Uganda in 1937. The nature of the virus was studied in Egypt in the 1950s. In 1957, as a result of an outbreak in Israel in the elderly, the WNV became recognized as a cause for severe inflammation of the spinal cord and brain in humans. In the early 1960s, it was first noted that horses were becoming ill in Egypt and France. This virus then emerged in North America in 1999, with encephalitis reported in humans and horses.

The virus cycles between the Culex mosquito and hosts such as birds, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The mosquito feeds on the infected hosts, carries the virus in its salivary glands, and then passes it on to humans or other animals during a blood meal. It usually takes
three to 15 days from the time of infection to the onset of disease symptoms. In the U.S., crows became a major host, but the disease would kill the crows; as a result, the number of infections dropped dramatically as the crow population died off.

West Nile encephalitis is
not transmitted from person to person (such as through touching or kissing or from a health-care worker caring for a sick person) nor can it be passed from animal to human. Blood transfusions are the exception; the virus may be passed among people by blood transfusions if the donor has been infected.

The chance that people will become severely ill from a mosquito bite is extremely small. According to the Centers for Disease Control and Prevention (CDC), even in areas where mosquitoes are reported to carry the virus, much fewer than 1% of them are infected. Furthermore, less than 1% of the people who get bitten and become infected will become severely ill. Therefore, the majority of cases are mild, and people can fully recover. For example, the CDC reported 690 people infected in 2011,with a total of 43 deaths.

Prognosis is usually guarded in the extremes of age (infants, young children, and the elderly). Death rates as a result of West Nile encephalitis range from 3%-15% and are highest among the elderly. At the present time, there is no documented evidence to suggest that a pregnancy is at risk due to WNV infection. It is assumed that if a person contracts WNV, he or she will develop a natural immunity that is lifelong. However, it may wane in later years. Currently, as of 2013, there is no commercially available vaccine for humans.

A relative of West Nile virus, Powassan virus, was discovered in 1958, but its vector is the blacklegged (deer) tick. It is very rare; only about 60 individuals have been diagnosed since 1958. There is no vaccine available. It has a 10% death rate.

La Crosse encephalitis: The first case occurred in La Crosse, Wisconsin, in 1963. Since then, the largest number of cases has been identified in woodland areas of the
midwestern and mid-Atlantic United States. This virus is the most common cause of mosquito-borne encephalitis in children younger than 16 years
of age. Each year, about 75 cases are reported to the Centers for Disease Control and Prevention (CDC). The virus cycles between the daytime-biting treehole mosquito (Aedes triseriatus) and hosts such as chipmunks and squirrels. Some investigators consider the cause of California encephalitis to be the La Crosse virus. The La Crosse encephalitis virus can cause adverse effects on IQ and school performance. About 80-100 people are diagnosed each year in the U.S.,
and 1% of people with this infection may die.

St. Louis encephalitis: Since 1964, an average of about 102 people is reported infected per year. Outbreaks can occur throughout most of the United States, although large urban epidemics have occurred in the
midwestern and southeastern regions. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974-1977. There were 2,500 cases in 35 states reported to the CDC. Most recently, there were 20 reported cases in New Orleans in 1999. The virus cycles between birds and the
Culex mosquitoes breeding in stagnant water. It grows in both the mosquito and the bird but does not make either one sick. Only the infected mosquito can transmit the disease to humans during the blood meal. The virus cannot be transmitted from person to person through kissing or touching nor can it be transmitted from the infected bird. The disease tends to affect mostly adults and is generally milder in children.